Mass spectrometer ion source with cooling means



MAss SPECTROMETEH 10N soURcE WITH COOLING MEANS Filed OCt. 26, 1954 R.M. ELLIOTT 5 Sheets-Sheet l Dec. 24, 1968 k UU 7///////////,M/ m

Defn 24, 196s R. M, ELLIQTT 3,418,513

MASS SPECTROMETER ION SOURCE WITH COOLING MEANS Fi1ed oci. 2e, 1964 ssheets-sheet e 7 F1' 5.5. Fi El l 55 5% r3@ 40 j52 3,5P n l 30 51j h t.f'f 52 I 54 55 56 L51 -f MASS SPECTROMETER ION SOURCE WITH COOLINGMEANS Filed Oct. 26. 1964 R. M. ELLIOTT f5 Sheets-Sheet 5 United StatesPatent O 3,418,513 MASS SPECTROMETER ION SOURCE WITH COOLING MEANSRichard Martin Elliott, Altrincham, England, assignor t AssociatedElectrical Industries Limited, London, Eng land, a British company FiledOct. 26, 1964, Ser. No. 406,607 Claims priority, application GreatBritain, Oct. 31, 1963, 43,063/63 9 Claims. (Cl. S13- 230) Thisinvention relates to ion sources in which a sample material introducedtherein is ionised. Such ion sources are used in a mass spectrometer inwhich the ions are subsequently analysed in a well-known manner so thatthe constituents of the sample can be determined.

Ions can be produced from a sample material simply by heating thematerial, but it is preferable for the sample to be converted to itsgaseous state and then bombarded with an electron beam in order toproduce-the ions. In the latter arrangement it is essential that ionsrepresentative of the constituent materials of the sample should beproduced in the ion source and to minimize generation of spurious ions.Therefore none of the molecules reachling the electron beam should havebeen decomposed nor should they have acquired a high thermal energy byreason of one or more previous collisions with a heated surface.

Ionisation Iof the molecules takes place in a chamber which may be aclosed box except for certain small open ings or a more open cagedefined by rods or bars, but which in any case is constructed of metaland is defined by a number tof metal surfaces. When an electron beam isemployed the ionisation chamber normally operates at temperatures in therange 50-250 C. This operating temperature can be varied by means of oneor more heaters surrounding or inserted into the ionisation chamberblock, but even when these heaters are switched off the operatingtemperature of the ionisation chamber will be from 50'-l50 C. becausethe presence of the ion source filament which produces the electronbeam, runs at about 2000 C. and is usually in good thermal contact withthe ionisation chamber. The operating temperature 0f the ionisationchamber produces heated metal surfaces on which the molecules of thesample may impinge before -being bombarded by the electron beam,resulting in either of the two unwanted conditions of the moleculesdescribed above.

An object of the present invention is to provide an improved ion sourcewhich is constructed so that the tend cncy for these adverse conditionsto be produced is reduced.

According to the present invention, a mass spectrometer ion sourcecomprises an ionisation chamber for ionising molecules of a samplematerial introduced into the source and cooling means by which thetemperature of the housing defining the chamber is reduced to a valueless than that which would exist in the absence of said means.

The cooling means may include means whereby a cooling fluid is broughtinto heat transferring relationship with the walls defining the chamber.In one embodiment of the invention ducting is arranged in thermalcontact with the walls of the ionisation chamber and cooling fiuid maybe passed through the ducting. Alternatively, the ducting may extendinto the chamber.

In a further embodiment of the invention the ionisation chamber is inheat transferring relationship with a heat sink, such as a body ofcopper, and ducting through which a cooling liuid is passed is inthermal contact with the heat sink. The cooling fluid in theseembodiments of the invention may, for example, be liquid nitrogen orwater.

In a still further embodiment of the invention a thermoelectric coolingdevice may be mounted on the wall of the ionisation chamber, oralternatively the cooling device can be in thermal contact with a heatsink which supports the ionisation chamber.

By lowering the temperature of the ionisation chamber to a requiredvalue the probability of a molecule being adversely affected -bycollision with a heated surface is reduced and therefore the efiicacy ofthe ion source in producing representative ions from the samplemolecules is increased.

The sample material is conveniently vaporised to produce the samplemolecules and the vapour may for eX- ample be produced at one end of atube containing the sample. The surfaces from which the sample materialis evaporated are independent of the surfaces defining the ionisationchamber and the sample molecules may make a number 0f collisions withthe walls of the ionisation chamber before being ionised or escapingthrough one of the openings in the chamber. If the temperature of themetal surfaces defining the ionisation chamber is higher than that ofthe surfaces from which the sample is evaporated, the molecules mayundergo further collisions with heated surfaces resulting in theundesirable effects described above.

On the other hand if the temperature of the surfaces defining theionisation chamber is lower than the evaporation temperature of thesample then there is a high probability that the sample molecules willcondense on their first collisions with the cooler surfaces. Ifdecomposition lof the molecules takes place during these collisions thedecomposition products will probably remain condensed and cannotsubsequently reach the ionising electron beam and result in unwantedions being produced.

Two useful ranges of temperatures of the surfaces defining theionisation chamber can therefore be defined.

(l) The range of temperatures above the temperature at which evaporationof the sample is taking place. This range will be useful for thosesubstances which are moderately stable with respect to collisions withmetal sur faces, but where decomposition and thermal energy are requiredto be kept to a minimum. Here the expected effects of the collisionswith the moderately cool surfaces are not serious and the molecules canbe allowed to reevaporate into the ionisation region; in fact it isdesirable that they should since if they remained condensed they wouldbe lost to the system and the effective sensitivity would be greatlyreduced.

This range of temperatures, which might be 0-300 C., is the mostgenerally useful.

(2) The range of temperatures below those at which evaporation of thesample is taking place. Decomposition products can be prevented fromreaching the ionisation region, but some sample molecules are alsoprevented from doing so and the sensitivity of the instrument istherefore reduced. This range would be used for those very unstablematerials where the expected effects of any collision are so serious(i.e. decomposition) that it is desirable that the molecules, decomposedor otherwise, should be prevented from re-evaporating even though thismeans a severe reduction in sensitivity. The result will be that only asmall proportion of molecules will reach the ionising region, but thosethat do will have arrived by line-of-sight paths without suffering anycollisions.

The range of temperatures involved might be 190 C. to C. The temperaturecontrolling means is therefore made flexible in its control so that thetemperature can be selected over the wide range covered by cases (l) andIn order that the invention may be more readily understood it will nowbe described by way of example with reference to the accompanyingdrawings; in which:

FIG. 1 is a side view partly sectioned on an axial plane of part of anion source embodying the invention;

FIG. 2 is a side view of part of an alternative arrangement to thatshown in FIG. l;

FIG. 3 is a plan view of the ionisation chamber shown in FIGS. l and 2;

FIG. 4 is a side elevation partly sectioned of an ion source inaccordance with a further embodiment of the invention;

FIG. 5 is a side view partly sectioned on an axial plane of part of anion source in accordance with a still further embodiment of theinvention; and

FIG. 6 is a side elevation of an ionisation chamber similar to thatshown in FIG. 3.

Referring to FIG. l, the part of an ion source which is illustratedcomprises an ionisation chamber 1 with means for producing an electronbeam for bombarding molecules of a sample material introduced into thechamber and a guide 2 is provided for the insertion of a probe carryingthe sample material into the correct position into the chamber. Thechamber is defined by a metal walled housing and is supported on plates3, 4 of good thermally conducting material which are attached to acooling block 5 acting as a heat sink. Block 5 is supported on anelectrically insulating frame 6 from an end plate 7 which together witha domed cover member 7 (part of which is shown) form the outer casing ofthe ion source which encloses the ionisation chamber 1. The ion sourcecan be evacuated through an opening 8 in the domed cover member 7.Insulating bushings 8 extend through the end plate 7 and supportconductors 9 for connection to the electrical components of the ionsource.

The ionisation chamber may be of any convenient form and a chamber whichis suitable for the purpose is illustrated in more detail in FIG. 3.This chamber, which is described by way of example, comprises a boxcontaining an electron gun assembly which comprises a filament 31 and anelectron accelerating electrode 32. The ionisation chamber also includesan ion accelerating electrode 34 displaced laterally from the electronbeam path and an electron trap electrode 30 behind a suitable aperturedplate 33. The box has side walls 35, end walls 36, a base 37 on whichthe ion accelerating electrode is supported, and an apertured lid whichis not illustrated. The electrons emitted from the filament 31 areaccelerated through the gap of the electrode 32 toward the trap 30 andthe electron beam is collimated by two permanent magnets (not shown)which are mounted on the inner surface of the casing of the ion sourceand located substantially coaxially with the electron beam.

The temperature control apparatus employed in the embodiment of theinvention illustrated in FIG. 1 comprises ducting in the form of twoconcentric metal tubes 11 and 12 which project through the end plates 7so that their inner ends extend into the heat sink 5. The inner end ofthe outer tube 12 is closed by a metal plate 13 and ts Within a tube 14of an electrically insulating, but thermally conducting, material suchas alumina. The tube 14 provides electrical insulation between the tubes11, 12, and the components of the ionisation chamber. Tube 14 litsclosely within the socket in the heat sink block 5. The outer ends oftubes 11, 12 are connected respectively to unions 15, 16 by means ofwhich a suitable cooling uid, for example liquid nitrogen or Water ispassed along the tubes. At the inner ends of the tubes there is a goodthermally conducting path between the cooling fluid and the block 5whereby the temperature in the block can be maintained at a requiredvalue.

A cap 17 is provided extending around the outer ends of the conductors 9and formed with a central aperture 18 through which the tubes 11, 12extend freely. A tubular cover 19 extends `around the tubes 11, 12 rfromthe aperture 18 within the cap 17 in order to prevent condensation fromforming on the outer surfaces of bushings 8'.

By suitably controlling the ow of the cooling iiuid through tubes 11, 12the temperature of the block 5 can be maintained at any required value,and since the ionisation chamber 1 is in heat transferring relationshipwith the block 5 through the supporting plates 3, 4 the temperature ofthe walls of the ionisation chamber can be controlled to a requiredIvalue.

In the alternative embodiment of the invention illustrated in FIG. 2 theonly difference is that the ducting extends into the ionisation chamber1 and is in good thermal contact with the walls thereof. The block 5'supports the ionisation chamber, but does not act as a heat sinktherefor, and the tubes 11, 12 pass through an opening centrally of theblock 5' and are not in contact with the block.

With this embodiment of the invention it is necessary that the tube 12should be made from an electrically insulating material in order toprovide the necessary electrical insulation between the tubes and theother components of the ion source.

FIG. 4 shows how the present invention can be applied to an ion sourcein which the ionisation mechanism is purely thermal, and in such an ionsource the reasons for wanting a low temperature in the ionisationregion are to reduce the number of spurious background ions fromresidual vacuum vapours (mainly hydrocarbons), and to obtain less ionswhich are produced from reevaporation of sample material which hadcondensed on the walls of the ion source during a previous analysis.

An ion source in which the ionisation is produced thermally is shown at20 and is supported on a metal sliding bar 21 which has a centralportion 22 of reduced diameter and upon which the ion source is mounted.The walls of the ionisation chamber forming part of the ion source arein heat transferring relationship with the portion 22 of the bar, and inaccordance with the invention the portion 22 of the bar is cooled duringthe operation of the ion source to reduce the temperature of the wallsof the ionisation chamber to Ia value less than that which will exist inthe absence of the cooling arrangement. A heat sink in the form of acylindrical body 23 has an arcuate groove 24 formed therein which is ofthe same radius of curvature as the periphery of the portion 22 of themetal bar 21 so that the heat sink can be located in good heattransferring relationship with the portion of the bar which supports theion source. The heat sink forms the base of a housing for containing thecooling fluid such as liquid nitrogen. The housing comprises a doublewalled portion 25 which is joined to the heat sink by flexible bellows26. The cooling uid in the housing lowers the temperature of the `heatsink 23 which in turn cools the walls of the ionisation chamber of thesource 20.

The source is located within an evacuated chamber 27, part of the wallsof which are provided by the portions of the bar 21 on each side of theportion 22 of reduced diameter, and to enable the ion source to berecharged the -heat sink is withdrawn from engagement with the bar bymeans of a mechanism including a cam 28, an operating lever 29 and a rod29' connecting the heat sink to the cam 28. The bar is then movedaxially to withdraw the ion source from the chamber 27.

In the embodiments of the invention described in connection with FIGS. lto 4 a cooling fluid has been employed, but in the embodiment of theinvention illustrated in FIG. 5 a thermo-electric cooling device 41 ismounted against the wall of t-he ionisation chamber 1. Electricalconnections 42 from the terminals of the cooling device pass through theend plate 7 of the ion source by means of insulating bushings 8. Byapplying a suitable potential to the terminals of the thermo-electricdevice cooling of the walls of the ionisation chamber can be broughtabout.

Referring to FIG. 6, radiation shields 40 are interposed between thefilament 31 of an ion source and the entrance to the ion chamber. Theshields in the form of thin plates each have an opening therein and theopenings are aligned with the filament 31 and an opening in theaccelerating electrode 32 through which the electrons enter into the ionchamber. The plates prevent much of the heat emitted by the filamentfrom falling upon the walls of the ionisation chamber. The plates areconnected in good thermal contact with the heat sink I5 (shown inFIG. 1) so that heat absorbed by the plates is rapidly conducted awayfrom the vicinity of the ionisation chamber.

What I claim is:

1. A mass spectrometer ion source including a metal housing defining achamber,

entry means for admission of vaporized sample material to the chamber,

an electron beam 4generator in the chamber for ionizing molecules ofvaporized sample material therein, said generator tending in use tocause heating of the housing to a temperature above the vaporizationtemperature of said sample material,

said housing also having Iat least one exit for ions generated therein,and

means for preventing thermal change in the molecules due to collisionwith a heated internal surface of the housing prior to ionization, saidmeans comprising means for cooling the metal housing toa temperaturesuiciently low as compared with the vaporization temperature of saidsample material to prevent such thermal change from occurring butinsufficiently low to cause condensation of all of the molecules there-2. A mass spectrometer ion source as claimed in claim 1 in which themeans for cooling the housing includes a tube of electrically insulatingthermally conductive material capable of conveying cooling Huid, whichtube is in electrically insulating thermally conductive relation withthe housing and projects into said chamber.

3. A mass spectrometer ion source as claimed in claim 1 in which saidmeans for cooling the housing comprises a thermo-electric cooling devicein heat transferring relationship with the housing, said device havingterminals connectible to a voltage source to bring about cooling of thedevice.

4. A mass spectrometer ion source including a metal housing defining achamber,

entry means for admission of vaporized sample material to the chamber,

an electron beam generator in the chamber for ionizing molecules ofvaporized sample material therein, said generator tending in use tocause heating of the housing to a temperature above the vaporizationtemperature of said sample material,

said housing 'also having at least one exit for ions generated therein,and

means for preventing thermal change in the molecules due to collisionwith a heated internal surface of the housing prior to ionization, saidmeans comprising a heat sink in electrically conductive heattransferring relationship with said housing :and means in electricallyinsulating heat transferring relationship with the heat sink for coolingthe heat sink and in consequence the metal housing to a temperaturesufficiently low as compared with the vaporization temperature of saidsample material to prevent such thermal change from occurring butinsufficiently low to cause condensation of all of the molecules there-5. A mass spectrometer ion source las claimed in claim 4 in which saidmeans for cooling the heat sink comprises ducting which is capable ofconveying cooling iiuid.

6. A mass spectrometer ion source as claimed in claim 5 in which saidducting includes a metal tube and a body of electrically insulatingthermally conductive material is disposed between the tube and the heatsink.

7. A mass spectrometer ion source as claimed in claim 6 in which saidbody is of alumina.

8. A mass spectrometer ion source as claimed in claim 4 in which saidelectron beam generator includes an incandescible filament, and at leastone apertured metal plate is positioned between said filament and partof said housing and is in heat transferring relation with said heatsink.

9. A mass spectrometer ion source including a metal housing defining achamber,

a heat source in the chamber for ionizing a sample of the materialintroduced into said chamber, said heat source tending in use to causeheating of the housing to a temperature above the vaporizationtemperature of said sample material,

said housing also having at least one exit Ifor ions generated therein,and

means for preventing thermal change in the molecules due to collisionwith a heated internal surface of the housing prior to ionization, saidmeans comprising means for cooling the metal -housing to a temperaturesufficiently low as compared with the vaporization temperature of saidsample material to prevent such thermal change from occurring butinsufficiently low to cause condensation of all of the moleculesthereon, and

said cooling means being a metal body in the form of a rod in circularcross section, said housing being mounted in a recess in said rod inheat transferring relationship with the wall defining the recess, theheat sink being engageable with a portion of the curved surface of therod and being provided with a curved co-engaging surface of the sameradius of curvature as the portion of the rod with which it engages.

References Cited UNITED STATES PATENTS 2,677,770 5/1954 Smyth 313-632,826,701 3/1938 Columbe 313-36 X 2,829,271 4/1958 Boucher 313-36 X2,932,753 4/1960 Arnott 313-109 3,142,752 7/ 1964 Hammer 313-633,217,162 11/1965 Wehner 313-63 3,226,542 12/ 1965 Craig 250-413,226,598 12/ 1965 Van Nimwegen 313-63 JAMES W. LAWRENCE, PrimaryExaminer.

R. L. JUDD, Assistant Examiner.

U.S. Cl. X.R.

1. A MASS SPECTROMETER ION SOURCE INCLUDING A METAL HOUSING DEFINING ACHAMBER, ENTRY MEANS FOR ADMISSION OF VAPORIZED SAMPLE MATERIAL TO THECHAMBER, AN ELECTRON BEAM GENERATOR IS THE CHAMBER FOR IONIZINGMOLECULES OF VAPORIZED SAMPLE MATERIAL THEREIN, SAID GENERATOR TENDINGIN USE TO CAUSE HEATING OF THE HOUSING TO A TEMPERATURE ABOVE THEVAPORIZATION TEMPERATURE OF SAID SAMPLE MATERIAL, SAID HOUSING ALSOHAVING AT LEAST ONE EXIT FOR IONS GENERATED THEREIN, AND MEANS FORPREVENTING THERMAL CHANGE IN THE MOLECULES DUE TO COLLISION WITH AHEATED INTERNAL SURFACE OF THE HOUSING PRIOR TO IONIZATION, SAID MEANSCOMPRISING MEANS FOR COOLING THE METAL HOUSING TO A TEMPERATURE